Interpretive Summary: Toxoplasma gondii is a single-celled parasite of all warm-blooded hosts worldwide. It causes mental retardation and loss of vision in children, and abortion in livestock. Cats are the main reservoir of T. gondii because they are the only hosts that can excrete the resistant stage (oocyst) of the parasite in the feces. Humans become infected by eating undercooked meat from infected animals and food and water contaminated with oocysts. This paper reports induction of dopamine by the parasite that might regulate host behavior. The results will be of interest to biologists, parasitologists, public health workers, and veterinarians.

Technical Abstract:
The common parasite Toxoplasma gondii induces behavioral alterations in its hosts including phenotypes increasing the likelihood of its transmission in rodents and reports of psychobehavioral alterations in humans. We have found that elevated levels of dopamine are associated with the encysted stage of parasite in brain tissue with concentrations as a punctiform pattern. The components of dopamine synthesis including a parasite-encoded tyrosine hydroxylase and dopa decarboxylase were found within the cysts. Infection led to increased dopamine release in a dopaminergic cell line with a direct correlation of parasite number with quantity of dopamine. Evidence of parasite produced dopamine defines a possible mechanism for parasite-induced behavioral changes and has important ramifications on infectious disease dynamics and implications for neurological disorders